This invention relates to full width array read or write bars and more particularly to such read or write bars assembled from components having materials with both high and low thermal coefficients of expansion that are coupled together in a manner to prevent internal thermal stress and consequent bar warpage while maintaining critical alignment therebetween.
It is well known in the reading and/or writing bar industry to assemble pagewidth raster input scanning (RIS) and raster output scanning (ROS) bars from relatively short RIS/ROS subunits placed end-to-end. Once assembled, the pagewidth RIS/ROS bars or scanning arrays have the requisite length and number of image processing elements to scan an entire line of information with a high image resolution. The subunits have either image reading arrays which comprise a succession of image sensing elements to convert a scanned image into electrical signals or image writing arrays which comprise a succession of light producing elements employed to produce latent images on image retentive surfaces or ink jet printheads to produce images from ejected ink droplets onto a recording medium, such as paper.
Typically, full width arrays experience significant thermal excursions during the fabrication process and during operation of such arrays. Thus, it is difficult to design each component in the package with low thermal expansion materials to match the thermal coefficient of expansion (TCE) of, for example, a silicon die containing either a plurality of image sensing elements or ink jet printheads. A printed wire board (PWB) capable of carrying high current, for example, has a relatively high TCE due to the dielectric material of the board itself and the high copper content retained on the board to carry current. In another example, the low cost polymers having relatively high coefficients of thermal expansion are used to manufacture ink supplying manifolds for thermal ink jet printheads. Finally, electrical connectors made from polymeric materials having the same high coefficients of thermal expansion are used to interconnect individual components comprising the complete array package. Consequently, few materials match the low TCE of the silicon die or the structural bar used in full width array thermal ink jet printheads. Therefore, when high TCE materials are coupled to low TCE materials, a thermal mismatch occurs that causes shear stress and strain during a thermal excursion. In the case-of a full width array thermal ink jet printbar or printhead, a thermal excursion takes place during the fabricating process (specifically, in the encapsulant cure process) that produces a printbar fabricated with built-in stress and warpage. Another thermal excursion occurs during the operation of a printbar, whereby the normal operating temperature exceeds the room temperature by approximately 30 degrees Centigrade. In either case, there is a tendency towards warpage of the printbar that causes a corresponding misplacement of marks on a copy sheet. Warpages on the order of 0.1 of an inch over an 11 inch wide printbar have been observed.
At the center of the fabricating process for full width array ink jet printheads is the unavoidable coupling of high TCE materials, such as printed wire boards and ink supply manifolds, with low TCE materials such as silicon die and a low TCE glass structural bar. Low TCE materials are critically important to the precision placement of marks on a copy sheet. High TCE materials are not dimensionally as critical, but instead support the function of the silicon die by providing the ink and electrical energy. It is important therefore, from a print quality standpoint, that the silicon and the low TCE glass experience minimal dimensional movement or warping during thermal excursions. The support function components, such as, for example, a printed wire board in the printbar assembly, can be more dimensionally variable than the printbar during thermal excursions because of the higher TCE, as long as the dimensional variability does not interfere with the support function. Thus, a printed wire board can expand or contract, and not interfere with the spot placement as long as it does not significantly affect the dimensions of the printbar and does not shift so much that the wire bonds between the two components are broken.
The prior art has failed to provide a means for fabricating a pagewidth scanning or imaging array that decouples high TCE materials from the dimensionally critical low TCE materials to prevent thermally induced stresses that cause corresponding bowing of or damage to the assembly.
U.S. Pat. No. 5,198,054 to Drake et al. discloses a process for fabricating reading and/or writing bars assembled from subunits, such as ink jet printhead subunits. At least two lengths of subunits are cut and placed on corresponding flat containers. An assembly robot places the subunits in a butted array on an alignment fixture and checks the positional error of the subunits as they are being assembled.
U.S. Pat. No. 5,160,945 to Drake discloses a pagewidth thermal ink jet printer. The printhead is of the type assembled from fully functional roofshooter type printhead subunits fixedly mounted on the surface of one side of a structural bar. A passageway is formed adjacent the bar side surface containing the printhead subunits with openings provided between the passageway and the ink inlets of the printhead subunits, mounted thereon so that ink supplied to the passageway in the bar maintains the individual subunits full of ink. The size of the printing zone for color printing is minimized because the roofshooter printhead subunits are mounted on one edge of the structural bar and may be stacked one on the other without need to provide space for the printhead subunits/or ink supply lines. In addition, the structural bar thickness enables the bar to be massive enough to prevent warping because of printhead operating temperatures.
U.S. Pat. No. 4,999,077 to Drake et al. discloses a method for fabricating a coplanar full width scanning array from a plurality of relatively short scanning subunits for reading and writing images. The subunits are fixedly mounted in an end-to-end relationship on a flat structural member with the subunits surfaces containing the scanning elements all being coplanar even though at least some of the subunits have varying thickness. This is accomplished by forming from a photopatternable thick film layer one or more keys on the subunit surface having the scanning elements and associated circuitry and positioning the keys into keyways produced from a photopatternable thick film layer on a flat surface of an alignment fixture. A conformal adhesive bonds a structural member to the assembled subunits to form the full width scanning array.
Xerox Disclosure Journal, Vol. 17, No. 5, pages 305-308, September/October 1992, discloses an encapsulation method for preventing damage to wire bonds between components in full width array devices having material with different thermal coefficients of expansion (TCE). The method comprises using two separate continuous beads of encapsulating material which encapsulates only the wire bond weld sites. This leaves the wires between the encapsulated weld sites free to move or flex without breaking or shearing the weld sites.